Muscular Reconstruction of the Forelimb of the 153 Theropod Dinosaur Mononykus olecranus Jada R. Smith, Sara H. Burch Department of Biological Sciences, SUNY Geneseo, Geneseo, NY

A A B C ABSTRACT

Mononykus olecranus was a small, nonavian predatory theropod that exhibited unusual forelimb size in comparison to other theropods. The forelimbs were distinctly stunted in size with only one finger and claw on each hand. The perplexity of these forelimbs has prompted many to study the function of the Mononykus forelimbs. This study uses phylogenetic muscle reconstruction methods along with previous reconstructions of theropods such as Tawa and Majungasaurus to help us better understand what B Mononykus may have been capable of. Overall, there are several modifications in the forelimb that improve the leverage of the shoulder flexors and elbow extensors. An improved lever arm for shoulder flexors like Supracoracoideus would be provided by the Figure 4. The left antebrachium (A and B) and manus (C) showing proposed muscle origins (red) and enlarged posteroventral process of the scapulocoracoid and the insertions (blue) in the lateral (A), medial (B), and dorsal (C) views. AN, Anconeus; APL, Abductor pollicis longus; AR, Abductor Radialis; BB, Biceps brachii; BR, Brachialis; ECR, Extensor carpi projecting deltopectoral crest of the humerus. Improved leverage for radialis; EDBS, Extensor digitorum brevis superficialis; ECU, Extensor carpi ulnaris; EDB, Extensor elbow extensors like Triceps brachii would be shown by the digitorum brevis; EA, Epitrochleoanconeus; FDLP, Flexor digitorum longus profundus; PT, Pronator Figure 2. The left scapulocoracoid showing proposed muscle origins (red) and insertions (blue) in the lateral teres; PA, Pronator accessorius; PQ, Pronator quadratus; TB, Triceps brachii. massive olecranon process of the ulna and enlargement of the (A) and medial (B) views. BB, Biceps brachii; CB, Coracobrachialis; DS, Deltoideus scapularis; DC, ectepicondyle of the humerus. The humerus also shows an Deltoideus clavicularis; LS, Levator scapulae; RH, Rhomboideus; SC, Supracoracoideus; SCA, Supracoracoideus accessorius; SS, Serratus superficialis; SHP, Scapulohumeralis posterior; SHA, enlargement of internal tuberosity related to the shoulder adductors. Scapulohumeralis anterior; SP, Serratus profundus; SBS, Subscapularis; SBC, Subcoracoideus; TR, CONCLUSIONS These reconstructions, along with the rest of the forelimb, are Trapezius; TBS, Triceps brachii scapularis. consistent with the hypothesis that Mononykus used its unusually A B The forelimb of Mononykus olecranus was reconstructed based on SBS + small forelimbs for digging or scratching, perhaps when foraging for SBC a phylogenetic muscle reconstruction method along with previous insects. BB information obtained from past reconstructions of the theropods CB Tawa and Majungasaurus. The reconstruction of a forelimb as distinctly stunted in size as Mononykus with only one finger on each TBM MATERIALS AND METHODS claw leaves a lot of ambiguity. However, the ability to compare this forelimb to past reconstructions gives way to a more unequivocal

The holotype of Mononykus olecranus (MPC 107/6; Perle et al., P BR reconstruction of the muscle attachment sites in the forelimb of 1994) preserves a complete forelimb including a scapulocoracoid, Mononykus. Our results suggest that Mononykus used this strange humerus, radius, ulna, manus. Our reconstruction of the PT forelimb to dig or scratch perhaps in an attempt to forage for musculature utilizes both phylogenetic muscle reconstruction insects. Evidence of this includes an enlarged posteroventral methods and information from previous reconstructions of FDLS FCU process located on the scapulocoracoid, a large olecranon process theropods such as Tawa and Majungasaurus (Burch, 2014; 2017). of the ulna along with an enlargement of the ectepicondyle located These previous theropod reconstructions provided the foundation PA EA on the humerus. The massive size of these allows for improved for the muscle attachment sites that were reconstructed in C SHA D leverage of the shoulder flexors (supracoracoideus) and elbow Mononykus olecranus. We used Adobe Illustrator to accurately DS extensors (triceps brachii). The improved leverage of these muscles trace a photo of the bone that could then be used in conjunction would allow for forceful digging motions.. Further studies and with the previous theropod reconstructions to identify possible SHP SC reconstructions on the forelimbs of theropods similar to Mononykus muscle attachment sites. The musculature of Mononykus was then will add more data to this reconstruction that may help resolve compared to that of other theropod dinosaurs, allowing for some ambiguity found within this reconstruction. assessment of previously hypothesized functions such as digging. DC TBM ACKNOWLEDGMENTS TBL Thank you to Dr. Burch for her guidance throughout the process of this reconstruction along with her

LD vast knowledge and experience. Collections access provided by Chinzorig Tsogbaatar, Mongolia Paleontological Center.

REFERENCES

Atuchin A. (n.d.) Mononykus. Natural History Museum. Retrieved from nhm.ac.uk Burch S. H. (2017) Myology of the forelimb of Majungasaurus crenatissimus (Theropoda, Figure 3. The left humerus showing proposed muscle origins (red) and insertions (blue) in the lateral (A), Abelisauridae) and the morphological consequences of extreme limb reduction. Journal of medial (B), posterior (C), and anterior (D) views. AN, Anconeus; AR, Abductor radialis; BR, Brachialis; BB, Anatomy 231, 1–17. Biceps brachii; CB, Coracobrachialis; DC, Deltoideus clavicularis; DS, Deltoideus scapularis; ECR, Extensor Burch S. H. (2014) Complete forelimb myology of the basal theropod dinosaur Tawa hallae based on carpi radialis; EDL, Extensor digitorum longus; ECU, Extensor carpi ulnaris; EA, Epitrochleoanconeus; a novel robust muscle reconstruction method. Journal of Anatomy 225, 271–297. FDLS, Flexor digitorum longus superficialis; FCU, Flexor carpi ulnaris; HR, Humeroradialis; LD, Latissimus Perle A., L. M. Chiappe, R. Barsbold, J. M. Clark, and M. A. Norell (1994) Skeletal Morphology of dorsi; PT, Pronator teres; P, Pectoralis; PA, Pronator accessorius; SC, Supracoracoideus; SU, Supinator; Mononykus olecranus (Theropoda: Avialae) from the Late Cretaceous of Mongolia. American Figure 1. Life reconstruction of Mononykus olecranus displaying its stunted forelimb SBS, Subscapularis; SBC, Subcoracoideus; SCA, Supracoracoideus accessorius; SHA, Scapulohumeralis Museum Novitates 3105, 1–29. with only one claw on each hand (Atuchin A., Natural History Museum). anterior; SHP, Scapulohumeralis posterior; TBM, Triceps brachii medialis; TBL, Triceps brachii longus.